Electrochemical gas sensors, with which the concentration of gaseous nitrogen compounds can be detected over a limited time period, are generally known. Such sensors are usually used in a great variety of industrial areas, ranging from the chemical industry to agricultural plants via the monitoring of refrigerating systems. They are used especially to recognize critical concentrations of flammable and/or toxic gases in time and to warn of a corresponding hazard. In particular, the monitoring of the concentration of ammonia (NH3), hydrazine and amines is of interest in this connection. Such electrochemical sensors usually comprise a plurality of electrodes, which are in conductive contact with an electrolyte liquid and form in this way a galvanic cell, hereinafter also called measuring cell.
EP 0 395 927 B1 discloses, for example, an electrochemical measuring cell for determining ammonia or hydrazine in a gaseous or liquid test sample, with at least one measuring electrode and a counterelectrode. To generate a reference potential for the determination of ammonia or hydrazine, a reference electrode, whose potential is used as a reference point for the measurement, is inserted into this measuring cell. EP 0 556 558 B1 also discloses such electrochemical measuring cells for determining ammonia, amines, hydrazine and hydrazine derivatives.
The detection of nitrogen-containing compounds, e.g., ammonia, different amines or hydrazine, is typically carried out in such measuring cells by means of an electrochemical reaction between the gas flowing into the sensor, the electrodes and the electrolyte of the sensor. For example, ammonia flowing in can be oxidized at a first electrode (typically called working electrode). Ammonium ions can be formed in the process, and they will diffuse to a second electrode (typically called counterelectrode). The ammonium ions can again be deprotonated there. This reaction leads to a detectable flow of current in the galvanic cell. The flow of current thus indicates the presence of the gas to be detected (hereinafter also called “reactive species”).
However, various problems may arise in such prior-art electrochemical measuring cells. Thus, not only ammonium ions, but also additional nitrogen compounds may be formed in the above-described reaction. However, these may become deposited on the electrodes and make difficult in this way the reaction of additional ammonia molecules or other molecules to be detected, which enter the sensor, and even block the reaction nearly completely. One also speaks of poisoning of the sensor (sensor poisoning) in this connection. Such a sensor poisoning may cause, on the one hand, an impairment of the basic measuring sensitivity of the sensor, and, on the other hand, the signal stability may decrease markedly in case of continuous gas admission. The sensitivity of the sensor may decrease further with each detection of gaseous ammonia, until reliable measurement will finally become impossible. In addition, changes of the zero signal in case of a change in the ambient humidity and the cross sensitivity to other gases may also be problematic.